The invention relates to a process for preparing terminally functionalized diene polymers and copolymers having reduced hysteresis. More particularly, the invention relates to a process for preparing terminally functionalized diene polymers and copolymers which comprises reacting a living diene polymer or copolymer obtained by anionic polymerization with certain substituted imines, to the terminally functionalized diene polymers or copolymers produced by the process, to elastomer compositions containing such diene polymers or copolymers and to tire treads formed from such elastomer compositions.
In recent years, those active in the tire industry have greatly increased their emphasis on the development of tires having both reduced rolling resistance and good wet traction properties. As is well known, that portion of the tire which exerts the greatest influence on rolling resistance and traction is the tread or tread rubber portion. Low rolling resistance is desirable from a fuel consumption standpoint while good wet traction is desirable from a safety standpoint. However, as a general rule, these properties have been found to conflict with each other. Thus, a reduction in rolling resistance generally leads to an almost directionally proportional reduction in wet traction while an increase in wet traction generally leads to an almost directionally proportional increase in rolling resistance.
The prior art has proposed a number of approaches to the solution of this problem. Such approaches have generally involved modifying the properties of the elastomer or elastomer composition utilized to form the tire tread in order to achieve the best possible balance between rolling resistance and traction. The approaches involving modification of the elastomer have generally been based on improving the interaction between the elastomer and the carbon black used in compounding the elastomer to prepare the tire tread composition in order to improve the dispersion cf the carbon black into the elastomer. This has the effect of reducing the hysteresis of the tire tread composition which in turn results in low rolling resistance.
One known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer compositions formed therefrom involves coupling the living diene polymer or copolymer chains with metal halides. Thus, U.S. Pat. Nos. 4,383,085 and 4,515,922 describe the coupling of living diene polymer or copolymer chains obtained by anionic polymerization using an organolithium initiator with metal halides such as tin halides, silicon halides and the like. These patents indicate that tire treads formed from rubber compositions containing the coupled polymers have reduced hysteresis along with reduced rolling resistance and improved we skid resistance.
Another known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer composition involves terminating the living diene polymer or copolymer chains with certain compounds containing functional groups which are reactive with the lithium terminals of the living polymer as illustrated by U.S. Pat. No. 4,835,209 and EPO 0316255. Thus, U.S. Pat. No. 4,835,209 discloses the termination of living diene polymer or copolymer chains with carbodiimides. The patent discloses that rubber compositions containing such polymers have excellent performance characteristics with respect to tensile strength, impact resistance, low heat-generating properties and wear resistance without impairing wet skid properties. EPO 0316255 discloses the termination of living diene polymer or copolymer chains with a capping agent selected from the group consisting of (a) halogenated nitriles having the structural formula X--A--C.tbd.N wherein X is a halogen atom and A is an alkylene group of 1 to 20 carbon atoms, (b) heterocyclic aromatic nitrogen-containing compounds, and (c) alkyl benzoates. This published application discloses that compositions containing such polymers have reduced hysteresis and that tire treads made from such compositions have lower rolling resistance and better traction characteristics.
Diene polymers and copolymers have also been subjected to various reactions with compounds containing reactive nitrogen-containing groups including certain substituted imines to improve certain properties of the polymers as disclosed by the following patents.
U.S. Pat. No. 3,178,398 relates to a method of preparing polymers including diene polymers and copolymers having terminal groups containing reactive nitrogen and to the curing of the resultant polymers with polyhalogen-containing compounds. The patent discloses that diene polymers or copolymers containing such terminal groups can be prepared by reacting the living diene polymer or copolymer with a non-polymerizable compound containing the structure ##STR2## Compounds containing the foregoing structure which are disclosed in the reference include heterocyclic nitrogen compounds, substituted imines and carbodiimides. Substituted imines which are specifically disclosed include N-ethylethylidenimine, N-methylbenzylidenimine, N-hexylcinnamylidenimine, N-decyl-2-ethyl-1,2-diphenylbutylidenimine, N-phenylbenzylidenimine, N-dodecylcyclohexanimine, N-propyl-2,5-cyclohexadienimine, N-methyl-1-naphthalenimine, N,N'-dimethylbutanediimine, N,N'-dipentyl-2-pentene-1,5-diimine, N-nonyl-1,4-naphthoquinonimine, N,N'-diphenyl-1,4-quinonediimine and N,N'-diphlenyl-1,3-indandiimine. The patent indicates that when such polymers are compounded and cured the resultant product has a good balance of physical properties. However, no mention is made of any effect on the hysteresis of the product.
U.S. Pat. No. 4,677,153 relates to a method for modifying a rubber having unsaturated carbon-to-carbon bonds (i.e. double bonds) with (a) an organic compound having a group represented by the formula --CH.dbd.N-- and (b) an organic acid halide having a group represented by the formula --COX wherein X is a halogen atom, in the presence of a Lewis acid. Organic compounds having the group represented by the formula --CH.dbd.N-- which are disclosed include substituted imines such as, for example, benzylidene methylamine, benzylidene aniline, dimethylaminobenzylidene butylaniline, etc. However, a careful reading of the reference indicates that the unsaturated rubber which is reacted with the (a) and (b) compounds is not a living polymer rubber but rather a previously terminated or "dead" polymer rubber. Thus, it appears clearly evident that the reaction between the unsaturated rubber and these compounds is not at the terminals of the polymer chains of the rubber.
The reference teaches that the modified rubber has improved green strength and when vulcanized has improved tensile and rebound resiliency.
U.S. Pat. No. 4,734,461 relates to a method of preparing diene polymers or copolymers, particularly block copolymers of the A-B-A type, which involves coupling living diene polymer or copolymer chains with ne coupling agents of the general formula (Y--CH.dbd.CH--CH.dbd.N).sub.n --X and/or ##STR3## wherein X=aryl, condensed aryl, aryl isolated by a heteroatom or by an alkenyl group, alkyl with a number of C atoms comprised between 2 and 20, cycloalkyl, alkylcycloalkyl, radical with heteroatom; all these groups possibly containing a functional group of vinyl aromatic, ester, aldehydic, ketonic, cyano type etc.; Y=aryl, cycloalkyl, alkyl, alkylaryl, alkylcycloalkyl, possibly containing a functional group of vinylaromatic, ester, aldehydic, ketonic, cyano type, etc; n=an integer of from 1 to 4; n'=an integer of from 1 to 4; R=H or alkyl group, preferably H; and Z=aryl, alkyl, cycloalkyl, possibly containing a functional group of vinylaromatic, ester, aldehydic, ketonic, cyano type, etc. The patent discloses that the resultant polymer has increased Mooney viscosity, molecular weight and green tensile strength as well as reduced cold flow.
U.S. Pat. No. 4,735,994 relates to a method of preparing diene polymers or copolymers, particularly block copolymers of the A-B-A type, which involves coupling living diene polymer or copolymer chains with new coupling agents of the general formula ##STR4## wherein R=alkyl, condensed aryl, aryl isolated by an heteroatom or by an alkenylic group, alkyl with 2 to 20 carbon atoms, cycloalkyl, alkylcycloalkyl, radical with heteroatoms; R'=aryl, cycloalkyl, alkyl, alkylaryl, alkylcycloalkyl; n=an integer of from 2 to 4, representing the functionality of radical R'"; R"=H or alkyl group, preferably H and R'"=aryl, alkyl or cycloalkyl. The reference discloses that the resultant polymer has increased Mooney viscosity, molecular weight and green tensile strength as well as reduced cold flow.
EPO 0207565 relates to a method of preparing diene polymers or copolymers, particularly block copolymers of the A-B-A type, which involves coupling living diene polymer or copolymer chains with new coupling agents of the general formula ##STR5## wherein R=aryl, alkyl, cycloalkyl; R'=aryl, alkyl containing 2 to 20 carbon atoms, cycloalkyl, condensed aryl, aryl isolated by an heteroatom or by an alkenylic group, radical with heteroatoms; n=an integer of from 1 to 4, representing the functionality of the R' group; R"=aryl, alkyl, cycloalkyl; n'=an integer of from 1 to 4, representing the functionality of the R" group and R'"=H or an alkyl group, preferably H. The reference discloses that the resultant polymers have increased Mooney viscosity, molecular weight and green tensile strength as well as reduced cold flow.
Coupling processes such as those described in this reference result in increased molecular weight of the polymer which lowers hysteresis. However, such increases in the molecular weight of the polymers tend to result in poor processability of rubber compounds containing the polymers.
U.S. Pat. No. 4,816,520 relates to terminally functionalized polymers, including diene polymers and copolymers, and a process for their preparation. The reference discloses that the terminally functionalized polymers are prepared from living polymers obtained by anionic polymerization of olefinically unsaturated monomers by first reacting the living polymers with capping reagents comprising various nitrogen compounds including substituted imines (Schiff bases) and diaziridines and then reacting the capped polymer with a terminating agent which contains halogen or acid anhydride groups. Capping reagents which are disclosed include among others a compound of the formula ##STR6## wherein R.sub.1 is H, alkyl, cycloalkyl or aryl and R.sub.2 and R.sub.3 are each alkyl, cycloalkyl or aryl. Terminating agents which are disclosed include halogen compounds such as chloromethylstyrenes, acryloyl chloride, methacryloyl chloride, epichlorohydrin, etc. and acid anhydride compounds such as acrylic anhydride, methacrylic anhydride, maleic anhydride, etc.